Engines, including diesel engines, gasoline engines, natural gas engines, and other engines known in the art, may exhaust a complex mixture of emissions. The emissions may include both gaseous and solid material, such as, for example, particulate matter. Particulate matter may include ash and unburned carbon particles generally referred to as soot. Emissions also may include compounds containing sulfur. For example, engine fuels, including diesel engine fuels, may contain sulfur which ultimately finds its way into an engine exhaust system from which it may be emitted to the environment.
Environmental concerns have resulted in the development of systems to treat engine exhaust. Some of these systems may employ exhaust treatment devices, such as filter systems that include particulate filters, to remove particulate matter from the exhaust flow. A particulate filter may include filter material designed to capture particulate matter, and may have an associated catalytic component. After an extended period of use, however, the filter material may become partially saturated with particulate matter, such as soot. This partial saturation may result in plugging to the point that backpressure on the engine is excessive and adversely affects engine operation.
A large portion of the collected particulate matter, e.g. soot, may be removed from the filter material through a process called regeneration. Filter systems may be characterized as passive filters or active filter, depending on the primary mode of regeneration. For example, in passive filters, a catalyst component generally may be incorporated to aid oxidation of particulate matter (e.g. soot), and heat required for oxidation may be provided by exhaust system temperatures. Regeneration of active filters may be accomplished by increasing the temperature at the site of the filter system periodically to oxidize particulate matter during a regeneration cycle. Particulate matter, e.g. soot, may be consumed by the heat of the regeneration process.
Various factors may affect particulate matter emissions and lead to increased accumulation of particulate matter within an aftertreatment element. For example, it has been found that particulate matter emissions may increase where fuel of high sulfur content is consumed by an engine. On the other hand, use of fuel of low sulfur content (e.g. ultra low sulfur diesel (ULSD)), may, in an otherwise properly operating engine system, not only result in less sulfur compounds being emitted to the environment, but also result in less particulate matter emissions. As another example, boost leaks in an engine system may alter expected air/fuel ratios and lead to increased particulate matter emissions. As yet another example, overcooling of the engine may lead to inefficient combustion and an increase in particulate matter emissions.
It is desirable that there be some effective manner for compensating for various factors affecting particulate matter accumulation within aftertreatment elements, such as fuel of high sulfur content, boost leaks, and engine overcooling, for example, and otherwise maintain particulate filter efficiency by accounting for such factors.
One system that addresses the effect of the sulfur component of engine fuel on exhaust aftertreatment is disclosed in European Patent Application No. EP 1 174 600 A2, published on Jan. 23, 2002 (“the '600 publication”). The '600 publication discloses an embodiment that includes a particulate filter for removing particulate matter, with the particulate filter including a NOx absorbent. The '600 publication discloses procedures and systems for addressing sulfur poisoning of the NOx catalyst (the poisoning being manifested as SOx) while avoiding thermal deterioration of the particulate filter. In one embodiment, the '600 publication separates a regeneration process to remove particulate matter from the particulate filter from a process for recovering NOx catalyst from sulfur poisoning. In another embodiment, the '600 publication controls an amount of fuel injected into the exhaust system in accordance with an estimated amount of particulate matter accumulation in order to consume the particulate matter before the higher temperature reduction process ensues for the sulfur poisoned NOx catalyst, thereby avoiding thermal deterioration.
While the procedures and systems of the '600 publication may address one factor affecting particulate matter accumulation within an aftertreatment element (i.e., sulfur poisoning), the systems of the '600 publication may be unduly complex to implement and operate reliably. In addition, the '600 publication focuses specifically on the issue of sulfur poisoning of NOx catalyst rather than a more generic approach that addresses a number of potential engine system failures. The '600 publication presumes, via a data map, that the NOx catalyst will be poisoned by sulfur and require the implementation of the procedures for alleviating the sulfur poisoning. The '600 publication does not take into account the factors that ultra low sulfur fuels may be employed normally, but that the engine may inadvertently or necessarily be fueled with higher sulfur containing fuel at times. Accordingly, the system of the '600 publication may lack the flexibility necessary to accommodate and compensate for various factors that may lead to an increased rate of accumulation of particulate matter in an aftertreatment element.
The disclosed methods and systems for maintaining aftertreatment efficiency are directed toward overcoming one or more of the problems set forth above.